This invention relates generally to lasers and, more particularly, to a high resolution spectral line selector for a waveguide laser that in operation introduces no coupling losses to a beam of laser radiation that exits a waveguide, propagates to a differaction grating, and is selectively reflected back into the waveguide.
A particular problem encountered in the design of lasers and, in particular, waveguide lasers is the energy loss associated with the coupling of the laser beam from an end of the laser waveguide to a mirror and back into the waveguide. This energy loss problem is also encountered when it is desired to select a particular spectral line for the laser radiation, as with a diffractive grating. Such energy losses are caused both by clipping of the optical field at the exit/entrance to the waveguide and by any mismatch between the amplitude and phase distribution of the returning field and that of the waveguide mode. If such a mismatch condition is present, the waveguide will act as a filter and attenuate the propagating field until the field matches the waveguide mode. This attenuation of the field results in an energy loss, which further results in a reduction in laser output power.
The amount of coupling loss as a function of mirror position for curved end mirrors with a waveguide laser having a circular bore was first characterized by R. L. Abrams in 1972 (IEEE J. Quant. Elect. OE-8, 838, Nov. 1972). Since 1972, square bore waveguide lasers have become more widely utilized but, in general, the design of the coupling optics has remained essentially the same.
FIG. 1 is adapted from the above mentioned journal article (p. 841) and shows the minimum possible coupling loss for an optimally curved single end mirror placed at a distance Z from the end of a circular bore waveguide. The parameter b is equal to 0.325 D.sup.2 /.lambda., where D is the diameter of the waveguide bore and .lambda. is the waveguide of the radiation that comprises the laser beam.
As an example, for a waveguide laser having a 2 mm diameter bore where .lambda.=10.6 .mu.m, b will equal 12.3 cm. It can be seen by reference to FIG. 1 that in order to achieve a coupling loss below 0.5% that the mirror must be closer than 0.37 mm or farther than 250 cm from the end of the bore. If 1.5% losses are acceptable, the mirror must be closer than 1.2 cm or farther than 120 cm or exactly at 12.3 cm. The additional 2% of round trip loss experienced by the laser beam (1.5% versus 0.5% at each end of the laser) may result in as much as a 40% reduction in a typical waveguide laser's output power. As may be appreciated, the physical construction of a particular waveguide laser or other factors, such as possible damage to the optics by the cavity plasma, may prohibit the placement of coupling optics at an optimal position, such as adjacent to the end of the bore. Thus, the coupling losses experienced by the laser may be prohibitively high. Also, waveguide lasers constructed with known coupling methods must rely entirely on waveguide losses to attain mode discrimination.
In addition to these coupling losses, for those lasers that employ a diffraction grating positioned relative to an exit of the waveguide in order to provide for spectral line selection, an additional energy loss due to the grating inefficiency is also experienced. Also, such laser/grating systems of the prior art provide only a slight alignment advantage for the desired spectral line, resulting in such systems often "hopping" to other spectral lines where the laser system may operate in other than the fundamental mode.